The present invention relates to radar systems and, in particular, to time-sharing radar systems and methods which are useful in alarm systems for detecting and avoiding potential automobile collisions.
A radar system, which is mounted on a vehicle such as an automobile and used in conjunction with an alarm system to detect and warn of potential frontal and rearend collisions, can be implemented using either a pulse radar system or an FM radar system. However, it has been recognized that FM radar systems are preferred over pulse radar systems because it is necessary for radar systems used in collision warning systems to have a minimum range of several decimeters. In FM radar systems, a frequency modulated (FM) signal having a frequency varying with time (and preferably varying linearly with time) is generated. In such systems, the generated FM signal is divided into two parts. One part is radiated from an antenna while the other part is supplied to one input terminal of a mixer as a local FM signal. The beam radiated from the antenna, if reflected by an object or target, will produce a return beam. The return beam may then be received by either the same or a different antenna and supplied to another input terminal of the mixer for mixing with the local FM signal. The mixer, in turn, will produce a beat signal having a frequency corresponding to the phase shift or time lag existing between the two signals supplied to its input terminals. The time lag is equal to the propagation time required for the radiated beam to travel to and return from the target. Thus, the lag time is proportional to the range to the target, and the range to the target can be calculated using the frequency of the beat signal.
To avoid interference from microwave transmission systems already in existence, it is presently preferred to use beams having a frequency higher than 30 GHz (i.e., beams having a wave length on the order of one millimeter:mm wave), which are rapidly attenuated upon propagation. This is so because the longest range to be detected is about several hundred meters. Beams having a frequency of about 60 GHz are exemplary.
FM radar systems are also preferred over pulse radar systems because a plurality of FM transceivers may be set to detect a direction to the target in addition to the range to the target. In such systems, a plurality of antennas are used and arranged to radiate beams having substantially the same pattern (or directivity) in slightly different directions from one another. In summary, FM signals of substantially equal amplitude are supplied to the antennas, beams of substantially the same amplitude are radiated by the antennas, and return beams are received by the same or other antennas. The direction to the target is calculated by processing ratio(s) of the amplitudes of the return beams. In systems of this type, there are two ways to avoid interference between the transceivers. One way is to allocate FM signals of different frequencies to each of the transceivers, and the other is to distribute FM signals of the same frequency to each of transceivers at different times. The latter method is referred to in the art as the time-sharing method, and the former is referred to in the art as the frequency-sharing method. The time-sharing method is generally preferred over the frequency-sharing method because the time-sharing method requires a reduced frequency range.
With regard to the issue of how to implement each of the transceivers, there are two ways. One way is to use a dedicated transmitting antenna and a separate dedicated receiving antenna. The other way is to use a single antenna, which is used both for transmitting and receiving (transceiving). In the latter case, a radiated FM signal beam and a received return beam are separated from each other using a circulator. The latter system, commonly referred to as a transceiving common antenna system, is presently preferred over the former system, which is commonly referred to as a dedicated antenna system, because the number of antennas required is reduced by one-half. This reduces the size and manufacturing cost of the overall system.
The use of transceiving antennas is especially important in FM radar systems wherein a relatively large number of antennas are required to detect a direction to a target in addition to a range to the target. Exemplary time-sharing/transceiving common antenna systems, which use mm wave FM signals, are disclosed in Japanese Patent Application HEI 2-303810, Japanese Patent Application HEI 3-42979, and U.S. Pat. No. 5,181,037.
In the prior art FM radar systems described above, a large number of antennas are required when it is desired to increase both the angular range of detection and the accuracy of such systems. As explained more fully below, this results in an increase in the size and manufacturing cost of the overall system.
The function of the prior art FM radar systems described above may be summarized as follows. For example, FIG. 4 illustrates the function of an FM radar system in which four beams Ba, Bb, Bc, and Bd, as shown in FIG. 4, are radiated from each of four transceiving common antennas A, B, C, and D (not shown in the Figure) respectively. The antennas A-D have the same radiation and receiving pattern (directivity), and the antennas A-D are arranged to radiate beams Ba-Bd in slightly different directions respectively. Further, assuming a target of a given size and position is shown as circle 100 in the figure, the amplitude of the return beam radiated and received by antenna B (Lb) (i.e., the return beam produced by radiated and reflected beam Bb) will exceed the amplitude of the other return beams. The amplitude of the return beam radiated and received by antenna A (La) (i.e., the return beam produced by radiated beam Ba) should be second highest. Finally, the amplitude levels of the return beams radiated and received by antenna C and D respectively (Lc, Ld) may both be zero. A direction to the target may be calculated using amplitudes La and Lb and the directions of antennas A and B.
To increase accuracy in a system of the type described above, it is desirable to increase the number of return beams having a non-zero amplitude. This is realized easily by reducing the difference of direction between adjacent beams (.delta..theta.) (for example, by reducing the setting angles between antennas). In this fashion, the amplitude level of return beam Lc radiated and received by antenna C may be converted to a non-zero value. Assuming that the directions of antennas A, B, and C are .theta.a, .theta.b, and .theta.c respectively, then a direction to the target .theta. can be calculated as follows. EQU .theta.=(La.multidot..theta.a+Lb.multidot..theta.b+Lc.multidot..theta.c)/(L a+Lb+Lc)
In this way, a direction to a target can be detected more accurately. However, the reduction of setting angles between the antennas leads to a reduction in the angular range of detection of the system. Accordingly, an increased number of antennas are required to increase both the accuracy and the angular range of detection of the system. This results in an increase in the size and manufacturing cost of the overall system.